The invention relates to a circuit arrangement with a reduction circuit for reducing interfering longitudinal voltages on a two-wire line, to which balanced push-pull signals can be applied.
In two-wire line systems, the signal voltage is defined as the difference between the voltages in the two cores relative to a reference-earth potential, such as earth. The longitudinal voltage of the two-wire line is understood to be the arithmetic mean of the two voltages. In the case of an ideal push-pull signal, the two voltages are completely balanced except for a constant, with the result that the longitudinal voltage vanishes or is constant over time. In the case of transmission systems which feed the two-wire line without balance-to-unbalance transformers, however, it is not possible to comply precisely with the requirement of temporal constancy of the longitudinal voltage at the switch-over moments and during a switching state. Overlap faults and fault pulses lead to system-dictated longitudinal interference voltages whose magnitude depends on the drive electronics used and the line network impedance. These longitudinal interference voltages can have such an interfering effect on capacitive vehicle antennas that expensive network screening measures or compensation of the longitudinal interference voltages become necessary.
DE 37 44 130 A1 discloses a circuit arrangement of the type mentioned in the introduction in which a reduction circuit for reducing interfering longitudinal voltages is looped into a two-wire line. The reduction circuit comprises a first and a second voltage divider each connected to the two cores of the two-wire line. The centre tap of the first voltage divider is connected to the inverting input of a differential amplifier, while the centre tap of the second voltage divider is connected to the output of the differential amplifier. The non-inverting input of the differential amplifier is at the balance potential, e.g. earth, of the two-wire line. By virtue of this circuitry, a negative feedback is obtained, that is to say the differential amplifier generates, in the two cores, a potential difference which is always opposed to the interfering longitudinal voltage.
In many two-wire line systems to which balanced push-pull signals are applied, such as e.g. in CAN bus systems of motor vehicles, an emergency running mode is provided for the case of a line fault, in which mode the message transmission is effected via just one core and is thus completely unbalanced with respect to earth. The signal voltage degenerates in this case into a very large longitudinal voltage whose amplitude, in the case of a CAN bus system, is e.g. 4 V instead of the 20 mV to 150 mV occurring in the two-wire mode. If the abovementioned circuit arrangement is used in such a transmission system for the purpose of suppressing the interfering longitudinal voltages, then although it effectively suppresses the interfering longitudinal voltages in the two-wire mode, in the single-wire mode it will attempt to suppress the emergency-running useful signal, since the latter has degenerated into a longitudinal voltage.
The article xe2x80x9cCANxe2x80x94das sichere Buskonzeptxe2x80x9d [CAN xe2x80x94the reliable bus concept] which appeared in the technical journal Elektronik 17/1991 describes such a fault-tolerant CAN bus on pages 96-101. In particular in the section xe2x80x9cMaBnahmen zur Behandlung globaler und lokaler Busfehlerxe2x80x9d [Measures for the treatment of global and local bus faults] and the subsequent sections, a description is given of how the CAN driver module detects a fault and of the fact that it is possible to switch over to single-wire operation in the event of particular faults. In order to detect all possible single faults, a program which identifies the setting and functional testing of all possible transmission modes is used as detector means. Interfering longitudinal voltages are not made a central theme in this citation.
The object of the invention is to specify a circuit arrangement of the generic type which, in addition to the two-wire mode with the capability of reducing interfering longitudinal voltages, also enables a single-wire mode of the two-wire line.
According to the invention, a circuit arrangement contains, in addition to a reduction circuit known per se, detector means connected to the two-wire line, which generate a switch-off signal in the absence of one of the two push-pull signals. Moreover, switching means connected to the detector means and to the reduction circuit are provided, which switch of f the reduction circuit when the switch-off signal is present.
Preferably, in the detector means also generate a switch-off signal when the interfering longitudinal voltages exceed a predeterminable threshold value. This prevents the reduction circuit from being overdriven, and the data signals from being unnecessarily distorted, in the event of high interfering longitudinal voltages.
This circuit arrangement can be realized in a particularly simple manner if, the detector means are designed as a CAN driver. Such conventional CAN drivers are already present as completed units and are used in CAN bus systems for detecting specific classes of transmission faults and, if appropriate, for initiating the switch-over to the single-wire mode. Either the CAN driver is additionally introduced into the system, or the switch-off signal is fed to the switching means by a CAN driver that is provided in any case.
An advantageous refinement of the invention allows interfering longitudinal voltages to be effectively suppressed even when the push-pull signals are only approximately balanced. This is achieved by virtue of the fact that, in the case of the reduction circuit described in the introduction, the non-inverting input of a differential amplifier has applied to it a low-pass-filtered summation signal as reference signal rather than a predetermined balance potential, for instance earth. Although this dispenses with the reduction of low-frequency components of the interfering longitudinal voltage, the reduction of the high-frequency components, which have a particularly adverse effect on message transmission, is considerably improved by this measure.